1. Field of the Invention
This invention relates to improved circuit breakers and, in particular, to new and improved solid state trip units for circuit breakers. Accordingly, it is a general object of this invention to provide new and improved devices of such character.
2. Background of the Invention
The traditional approach to sensing electrical overloads by a circuit breaker has been through a combination of thermal and magnetic action. Current, which passes through the circuit breaker, is caused to flow through a composite material of two or more layers of metal with controlled expansion and resistance, such material being referred to as bimetal or trimetal. Such current causes heating of the bimetal or trimetal at a rate given by the power dissipated, expressed as I.sup.2 R. Upon heating, the composite material deflects due to the different expansion rates of its constituent layers. Such deflection produces the necessary mechanical force to trip the circuit breaker. When a large overload current is drawn through the circuit breaker, magnetic forces, generated by the load current, are sufficient to attract a ferromagnetic armature, much in the manner of a relay closing, to provide mechanical force to trip the breaker.
The foregoing combination of a dissipated powertime relationship (I.sup.2 Rt) has proven effective to protect building wiring for moderate overloads, and the magnetic trip assures much more rapid disconnection of large overloads. Though it is sometimes referred to as "instantaneous" trip, this is not truly accurate. Electronic means for sensing and control have been developed in the prior art which are inexpensive, which offer additional time classification such as short term, and which offer field adjustability of sensitivity settings.
Disadvantageously, some of the electronic trip units of the prior art do not provide the desired I.sup.2 t powertime characteristic, and have possible circuit drift characteristics which could result in inaccurate or false tripping performance.
Various prior art electronic sensing systems utilized analog means for determining tripping time. Current squared time dependence was achieved by gating one or more resistance-capacitance networks with different time constants as a function of the current signal amplitude, and charging a fixed capacitor until it reached some fixed amplitude. Disadvantageously, even beyond the complexity of such multi-RC network approach, minor shifts in component values caused serious shifts in trip timing, resulting in possible damage due to not tripping soon enough or in nuisance from tripping too soon.